Microalgae have the ability to accumulate significant amounts of lipids, primarily in the form of triacylglycerol (TAG), especially under stress conditions like nutrient limitation, temperature, pH, or light stress. Nitrogen deprivation is considered a critical factor affecting lipid metabolism in microalgae. Nitrogen deprivation limits amino acid production and decreases protein synthesis, thereby impairing growth and photosynthesis, which leads to an accumulation of lipids, in particular TAG, which are used as carbon and energy provisions.
The ability of microalgae such as Phaeodactylum tricornutum to accumulate TAG has triggered their exploitation as host for fatty acid production, e.g. for biofuel production, for chemical applications or in food industry. P. tricornutum for instance is currently used for the industrial production of omega-3 polyunsaturated fatty acids.
Approaches to enhance TAG accumulation can rely on nutrient starvation such as nitrogen starvation, in particular the reduction of nitrate (NO3−) availability in the medium. Disrupting the assimilation pathway of NO3− by genetic engineering has therefore been considered as a way to trigger TAG accumulation, and reducing the expression of a nitrate reductase from P. tricornutum has been shown to promote TAG accumulation per cell (Levitan et al. 2015 Proc Natl Acad Sci USA 112:412-417, US 2012/0282676). Other attempts to promote TAG accumulation include the stimulation of fatty acid and TAG biosynthesis, the blocking of pathways that divert carbon to alternative metabolic routes and eventually the arrest of TAG catabolism through genetic engineering of the microalgae (Maréchal 2015 In Techniques de I'Ingénieur. In 186: 1-19 and US 2014/0256927). Various strategies can also be combined.
The implementation of microalgae in industrial processes is currently based on a two-step process: first, the biomass grows using the nutrients provided in the culture medium and second, growth is slowed down or stopped by nutrient starvation, e.g. nitrogen starvation. In the above second step, TAG accumulation occurs, whereas there is virtually no TAG accumulation during the first cell growth step.
There is a need for alternative methods for enhancing triacylglycerol accumulation in microalgae, preferably without compromising cell growth and biomass yield so as to improve overall lipid productivity. In other words, it would be advantageous to implement a method in which TAG can accumulate during a cell growth step.